By the year 1991, the particulate emission standards set by the Environmental Protection Agency (EPA) will require all urban buses to emit less than 0.1 gm/hp-hr of particulate matter. The same standard will apply to heavy duty trucks in 1994. Particulates are defined by the EPA as any matter in the exhaust of an internal combustion engine, other than condensed water, which is capable of being collected by a standard filter after dilution with ambient air at a temperature of 125 degrees Fahrenheit. Included in this definition are agglomerated carbon particles, absorbed hydrocarbons, including known carcinogens, and sulfates.
These particulates are very small in size, with a mass median diameter in the range of 0.1-1.0 micrometers, and are extremely light weight. Particulate filter traps have been developed which are effective to remove a sufficient quantity of the particulates from the exhaust gas of a typical diesel engine for a truck or bus to bring the exhaust emissions into compliance with the EPA regulations. During normal operations of a typical vehicle engine, approximately 20 cubic feet of particulate matter must be trapped per 100,000 miles of vehicle operation. Obviously this particulate matter cannot be stored within the vehicle. Therefore successful long term operation of a particulate trap-based exhaust aftertreatment system (EAS) requires some method for removal of the trapped particulates. One method which has proven to be successful has been to provide means to burn off the trapped particles to regenerate the filter. See for example Mogaka et al., "Performance and Regeneration Characteristics of a Cellular Ceramic Diesel Particulate Trap," SAE Paper No. 82 0272, published Feb. 22-26, 1982. The regeneration process is typically initiated by a control system and is carried out by the delivery of heat to the inlet of the particulate trap at a temperature in excess of 1200 degrees Fahrenheit. The process results in oxidation of the filtered carbonaceous particulates in a manner that restores the trap's "clean" flow restriction but unavoidably produces temperature gradients and resultant thermal stresses in the particulate trap. The magnitude of these stresses must be controlled to a level that will not result in fatigue failure of the filter within its designed operating life.
A number of factors influence the magnitude of the stresses such as regeneration gas flow rate, oxygen concentration, and trap inlet temperature distribution, all of which are determined by physical characteristics of the system design and hence are "fixed." The single most important, non-fixed factor in determining the magnitude of these stresses, and hence the life of the system is, the mass of particulates that is allowed to accumulate in the trap before the control system actively initiates the regeneration process. Should a trap be allowed to become excessively loaded with particulates, it can be predicted that, upon regeneration by burn off, deleterious thermal stress fatigue will result.
One solution to the problem would be to burn off the trapped particulates at very frequent intervals but such a technique would be wasteful of the fuel needed to promote the burn off, and be partially self defeating of the ultimate purpose of particulate emission reduction since during burn off, the engine exhaust gases may be in some systems released to the atmosphere without filtration. Obviously, a need exists for determining as accurately as possible the mass of particles actually trapped but this need must be weighed against the expense and complexity of the sensing means used to make the determination. It can thus be seen that the means of determining the mass of particulates in the trap and therefore the means of deciding "when" to regenerate is by far the most crucial aspect of the control system.
Fundamental to the design of a control system is the choice of physical parameters which are to be sensed to determine trap "loading" which in turn is dependent on the selection of a quantifiable parameter representing the degree to which a filter is loaded with particles. One approach has been to define a dimensionless parameter (M) equal to the ratio of the dimensionless pressure drop (pressure drop divided by the kinetic pressure, 1/2.epsilon.V.sup.2) across a loaded trap to that across a clean trap at the same Reynolds number, i.e. ##EQU1## wherein V=flow rate of exhaust gas stream
.rho.=gas Density PA1 Re=Reynolds number based on mass flow rate and fluid viscosity PA1 .DELTA.P=differential pressure signal across the trap PA1 V=volume flow rate (or velocity) of the exhaust gas stream at the trap inlet PA1 C, D=predetermined constants empirically derived.
Obviously, M=1 for a clean trap and is greater than 1 for a loaded trap. See pages 70 and 71 Magaka et al, supra. Measurement of the pressure drop across a loaded trap is fairly straight forward but direct measurement of the pressure drop across the same filter trap, having no particulates trapped therein but operating under identical flow conditions, is not possible and must be derived indirectly. One theoretical approach for estimating the clean, dimensionless pressure drop is disclosed in the Magaka et al, page 87, supra as follows: ##EQU2## wherein k and x=empirically derived constants
Direct measurement of these physical quantities are difficult so that indirect measurements have been used based on the speed density law using engine displacement, RPM, intake manifold pressure and temperature and engine volumetric efficiency information.
Attempts have been made to simplify the number of actual physical measurements required in a particulate regeneration control system such as disclosed in U.S. Pat. No. 4,608,640 to Shinzawa. Shinzawa '640 utilizes the pressure drop across the filter trap and a variable limit determined as a function of the inlet trap pressure. This approach ignores gas viscosity variation which is quite large over the normal operating range of a diesel engine. Similarly flawed approaches are disclosed in U.S. Pat. Nos. 4,630,438, 4,603,550, and 4,567,725 to Shinzawa. In another Shinzawa patent (U.S. Pat. No. 4,610,138) the pressure ratio across the trap is used as an indication of trap loading. While possibly of greater utility, this approach is still an incomplete picture of the true relationship between the flow parameters and mass loading.
The patent to Tagaki (U.S. Pat. No. 4,492,079) plots trap differential pressure vs. trap outlet pressure in a system that does not simply discharge the trap into the atmosphere but rather flows the discharge through a length of pipe and then through a muffler. In effect, this uses the piping and muffler downstream of the trap as a flowmeter without any means for accounting for temperature effects. This is therefore extremely installation specific and very cumbersome to implement.
Another approach has been to construct a physical analog of a clean filter such as disclosed in U.S. Pat. No. 4,544,388 to Rao et al. wherein an open channel, honeycomb structure is placed upstream of the actual filter. This honeycomb structure is said to have a porosity which is much larger than the filter trap so that none of the particulates will become entrapped. While simple in concept, physical analogs of the type disclosed by Rao et al. suffer a number of disadvantages. For example, the large porosity necessary to insure that no particles become entrapped inherently causes the structure to be an inaccurate analog of the filter trap. Moreover, placement of the analog structure upstream of the actual filter trap renders the system susceptible to serious error should an exhaust leak occur anywhere between the analog structure and the downstream side of the filter trap.
A need therefore exists for a simple, inexpensive, yet highly accurate and reliable particulate trap regeneration control system which overcomes the prior art deficiencies noted above.